Effects of Subsurface Topography and Hydrogeology on Gunung Talang Hot Water Systems, Sumatra, Indonesia: an Analysis Based on Gravity Data
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PROCEEDINGS, Thirty-Sixth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, January 31 - February 2, 2011 SGP-TR-191 EFFECTS OF SUBSURFACE TOPOGRAPHY AND HYDROGEOLOGY ON GUNUNG TALANG HOT WATER SYSTEMS, SUMATRA, INDONESIA: AN ANALYSIS BASED ON GRAVITY DATA Alanda Idral Indonesian Geological Agency-Center for Geological Resources Jalan Soekarno-Hatta No.444, Bandung, 40286-Indonesia [email protected] or/[email protected] ABSTRACT Administratively Gunung Talang geothermal field lies in Solok Regency- West Sumatra Province. Low gravity anomaly (bouguer and residual) occured around Cupak and Gabuo hot water complexes, whilst high gravity anomaly (bouguer and residual) are found in the south SW of the survey area (around Mt. Talang) and is considered as the heat source of the geothermal system in the area. The gravity anomaly leneament trending NW-SE coincident with regional trend of Sumatra Fault. A-B and C-D x- sextion show concave-like shape and getting deeper to the north, this condition reflect the hydrogeology in the area flow from the south to the north and caused the dillution of hot water in the north (normal pH), whilst to the south the hot water has acid type (acid pH). Figure 1; Survey area INTRODUCTION GRAVITY METHOD In geothermal exploration, gravity method is In geothermal exploration the gravity method is one of many geophysical methods that is used to used to delineate the sub surface structures that delineate the sub surface structure of geothermal control the geothermal system. The geothermal system. Principally the method observes the heat source and the accumulation area lateral variation of densities. Because the underneath cause the densities difference densities differnce are relatively small, in order --8 between rocks and its surrounding. The of 10 , therefore its needed to have the very difference is used as a reference for gravity high sensitivity equipment. survey to know the effect of unknown source for The gravity data that is used in the paper came the variation of gravitational value. The variation from gravity data of Cupak-Batubajanjang hot of gravity values are influenced by latitudes, water complex, Solok-West Sumatra Indonesia, tides and topographic effects, therfore, to know which have been carried out by DIM, a the variation we have to calculate or reduce the goverment agencies, in 2004. vakue to those factors Administratively the Cupak-Batubajanjang hot The gravity data was collected from 5 gridding water complex, belong to Solok Regency, West and random lines, iwhich the observe point Sumatra Province, Indonesia ( figure.1 ) distance are 250 – 500 m. The data were collected using La Coste & Romberg gravitymeter G type. MT. TALANG GEOLOGICAL REVIEW Geologically the area is covered by Pra Tertiary – Quartenary age of rokcs that are composed by (DIM-2004) Metamorphic rocks, Trts (phylite), Old volcanic, QTau, (andesite, tuff breccia), lava volcanic, QLv (andesit, tuff breccia, piroklastic) and surface deposit, QLh. Generally, metamorphic and volcanic rocks are weathered, in some places the volcanic rocks are also altered, and characterised by sericite and chlorite. In addition on volcanic rocks collumnar and sheet joints occured caused the rocks have a good porosity and permeability, therefore, it is a good media for transfering surface water into Figure 2: Bouguer gravity anomaly a geothermal system in the area. Regional Garvity Anomaly GEOLOGICAL STRUCTURES Regional gravity anomaly (figure 3 ) shows the Faults and rings (result of volcanic eruption) are contour liniation trending nw-se, which is common in the area. The faults are normal and related to regional fault structures of sumatra. trending NW-SE, N-S and NE-SW. Among the Low regional gravity anomali (<212 mgal) faults Batubajanjang fault is important as its dominated the central part of the survey area and control the geothermal manifestation in elongated from n nw to s se. G.Talang to the surface. Medium regional gravity anomaly ( 212 – 220 mgal) flanks low regional gravity anomali in the sw and ne of the survey area. GRAVITY RESULTS Such as medium anomaly, high regional gravity Variation of rock density inthe area vraies from anomaly (212 – 220 mgal) is also surrounded 2.13 (phylite) – 2,66 (andesite) gr/cm3. 2,61 both low and medium regional anomaly in the gram/cm3 rocks density is used for calculating sw and ne end of the survey area. the gravity data in the area. Bouguer Anomaly Three zone of bouguer anomaly is found in the area, (figure 2.) namely: Low bouguer anomaly( < 214 m.gal), covering Cupak hot water complex, is consider associated with altered and fracture rocks of lava andesite and faults. Medium bouguer anomaly (214 – 223 m.gal) is related to fresh old volcanic andesite, breccia rocks and tuff. High bouguer anomaly (> 223 m.gal) is associated with fresh metamorphic (phylite) rocks. Fault structures are characterized by tightly, straight and turn line of contour density. Figure 3 : Regional Gravity anomaly Residual Gravity Anomaly The Present Of Ne-Sw Fault Structures That Are Related To Big Regional Sumatra Faults (Semangko Faults) Are Clarified By Dominantly Ne-Sw Contour Lineament (Figure 4), Whilst Minor Structures Are Characterized By Turn A-B cross section shows concave-like shape, Line Of Contour Density And Negative Or inwhich the density contras. varies from 0.005 Positive Poles. to – 0.12. The x-section is composed of lava Low (Negative) Residual Gravity Anomaly (< - andesite and secondary deposit that have 5 M.Gal To > - 5 M.Gal) Covers Both Cupak different level of alteration and fracturing. The And Gabuo Complexes, And Trending Nw-Se. thickeness of the rock or the depth of concave- The Anomly Is Considered Associated With like shape is > 3500 m thick. The x-section is Altered And Fractured Zones Of Andesite Rock also show fault structures that is trending NW – In The Area. SW and interpreted as part of Sumatra Fault High (Positive) Residual Anomaly ( 0 - > 5 systems, (se figure 5). M.Gal) Occurred In The North Ne And South Sw Of The Survey Arera Surrounding Low Residual Gravity Anomaly In The Middle. High NE Positive Anomaly In The Sw Around Mt.Talang Muda (Mt.Jantan), Holocene Age, Is Considered A The Heat Source Of Geothermal System Of G.Talang. The Area Is Covered By Andesit And Breccia Tuff Of Bukit Bakar; And Andesitic Lava Of Gunung Batino-Jantan. Figure 5: 2-D model A-B,x-section B C-D x—section is also show concave-like shape with the depth of < 2500 m, whilst density contras, vary from – 0.12 to 0.105. The rocks in the area is also composed by lava andesite and A secondary deposit that have different level of D alteration and fracturing. The x-sectin is also show the exitency of NW-SE trending faults, (figure 6). C Figure 4: Regional Gravity anomaly Structures The Leneament Of gravity anomaly (bouguer, regional and residual anomalous) trending NW- SE are coincident. with main regional fault structure of Sumatra ( Sumatra Fault) The Gunung Talang fault system that moves laterally is considered to be the part of Sumatra fault. Figure 6: 2-D model C-D,x-section Mean while, bouguer anomalous, in some places show wrench faults pattern that indicated a fracture zone. The zone is identical to secondary CONCLUSION permeability that cause reservoir zone to occured Low gravity anoma;ly (bouguer and residual) (see figure 2 and 4) around Cupak and Gabuo hot water complexes is associated with the alteration zone whilst high 2-D Model gravity anomaly (bouguer and residual) in the Two cross sections, A-B ( figure 5 ) and C-D ( south SW of the survey area is interpreted as the figure 6..), trending NE –SW are built up heat source of the geothermal system in the area crossing Cupak and Gabuo hot water, 3 Based on A-B and C-D x-sections, the respectively. 2.61gr/cm . Density is used for subsurface topografi in the survey area is modelling. getting shallow to the south, therefore the deep- water flow from the south to the north. This Dit.Inventarisasi Sumberdaya Mineral (2002) : condition caused the hot water in the north Penyelidikan Terpadu Daerah Panas Bumi Adum, (Cupak complex) has normal pH, whilst to the Kab.Lembata-NTT, Tidak Dipublikasikan. south (Gabuo complex) has acid pH. ( dillution Dit.Inventarisasi Sumberdaya Mineral (2002) : of hot water by surface water occured to the Penyelidikan Terpadu Daerah Panas Bumi north) Lesugolo,Kab.Ende-NTT. Tidak Dipublikasikan. REFERENCES Komazawa..M., Et Al., 2002: Gravity Anomalies Of The Central Flores Island Indonesia. Special Akbar, N., 1972. Inventarisasi dan Penyelidikan Publication: Indonesia-Japan Geothermal Pendahuluan Terhadap Gejala-Gejala Panas Expl.Project in Flores Island. 211-223 Bumi di Daerah Sumatera Barat, Direktorat Geologi Silitonga Dan Kastowo., 1995, Edisi 2, Peta Geologi Lembar Solok, Sumatera Barat Skala Dit.Vulkanologi (2000): Penyelidikan Terpadu 1:250.000. Pusat Penelitian Dan Pengembangan Daerah Panasbumi Atedai, Kab.Lembata- Geologi. Bandung. Lomblen-NTT. Tidak Dipublikasikan Telford, W.M. Et Al, 1982. Applied Geophysics. Cambridge University Press Cambridge .